This invention relates generally to the smoothing of conductive films, and, more particularly to the smoothing of thin conductive films by GCIB.
Thin film magnetic sensors based on magnetoresistance (MR) effects have come to play an important role in the implementation of high data-density read sensors for hard disk drive magnetic information storage technology. Such devices are often referred to as MR read heads. They are typically capable of reading data stored at higher densities than was possible with inductive read heads. MR sensors detect information bits stored as magnetic field changes by responding with a change in resistance dependent on the sensed magnetic field and field direction.
It is known (see for example R. White, xe2x80x9cGiant Magnetoresistance: A Primerxe2x80x9d, IEEE Trans Magnetics, 28(5) (September 1992), p 2482-87) that improved sensors based on a giant magnetoresistance (GMR) effect can be more sensitive than basic MR sensors by employing a xe2x80x9cspin valvexe2x80x9d (SV) configuration which exploits the quantum nature of electrons (two allowed spin directionsxe2x80x94up and down). In such devices, electrons with a spin direction aligned with a ferromagnetic material""s magnetic orientation move through the material freely, while those with a spin direction opposite that of the material""s magnetic orientation undergo more frequent collisions with atom of the material and experience higher electrical resistivity. Various SV designs have been developed to optimize the sensitivity and noise levels to achieve denser read head capabilities. Such. devices are fabricated as a series of stacked thin films of selected materials having selected properties and deposited on a substrate material. One figure of merit of a SV is the GMR ratio (xcex94R/R), the percentage resistance change for a predetermined change in the magnitude or direction of an external magnetic field. A higher GMR ratio means that the device is a more sensitive transducer.
The most elementary SV designs comprise two thin ferromagnetic films of differing magnetic orientation separated by a thin third, conductive but non-magnetic, layer. For best performance, the ferromagnetic films and the interface film must be quite thinxe2x80x94on the order of the mean free path of the electrons, several tenths of nm to tens of nm (several Angstroms to hundreds of Angstroms). In one of the two ferromagnetic layers, the magnetic field direction is held fixed (pinned) in orientation. In the other ferromagnetic layer, the field is initially of different orientation from that of the pinned layer, but is free to change orientation in response to an external magnetic influence. When the free layer is brought into magnetic alignment with the pinned layer by the influence of an external magnetic field, there results a change in electrical resistivity of the system, which permits transduction of the magnetic change into an electrical change.
There are two commonly employed methods of pinning the magnetic orientation of one of the layers. In one approach, an additional layer of antiferromagnetic material is located adjacent to the layer to be pinned. In the other method, the two ferromagnetic layers are fabricated of materials having differing coercivities, that of the pinned layer being substantially higher than that of the free layer so that the magnetic orientation of the pinned layer is not influenced by the external field, but that of the free layer is. In both approaches, the devices are normally constructed on a planar substrate using thin film deposition techniques and planar processing techniques that are generally well known in the industry.
The substrate is frequently an insulating material like glass or ceramic or is a conductive material, such as silicon, having a dielectric surface film, such as SiO2. One role of the insulating film or substrate is to assure that the substrate conductivity is low enough to avoid an undesirable electrical shunting effect on the magnetoresistive layers.
For hard disk drive heads it is often common practice to fabricate a SV read head in close juxtaposition with a thin film inductive write head.
The GMR ratios of fabricated devices.are dependent on many parameters including the materials and the thicknesses of the layers. From published information, it is also known that the GMR ratios of SV and other GMR devices benefit from having a controlled smoothness and flatness of the magnetic and interface layer surfaces (see for examples, Choe and Steinback in xe2x80x9cSurface roughness effects on magnetoresistive and magnetic properties of NiFe thin films, J. Appl. Phys., 85(8) (1999), pp 5777-9 and Kools et. al. in xe2x80x9cEffect of finite magnetic film thickness on Nxc3xa9el coupling in spin valvesxe2x80x9d, J. Appl. Phys., 85(8) (1999), pp 4466-8). Thus it is necessary to control, among other parameters, the thickness, smoothness, and flatness of the films. This is also true for other types of GMR effect devices, examples being given by Schad et.al. in xe2x80x9cInfluence of different kinds of interface roughness on the giant magnetoresistance in Fe/Cr superlatticesxe2x80x9d, J. Mag. and Mag. Matls., 156 (1996), pp 339-40, as well as by Ben Youssef et. al. in xe2x80x9cCorrelation of GMR with texture and interfacial roughness in optimized rf sputtering deposited Co/Cu multilayersxe2x80x9d, J. Mag. and Mag. Matls., 165 (1997), pp 288-91.
For this reason, during the fabrication of GMR devices, it has been the practice to employ chemical-mechanical polishing techniques for smoothing and planarizing the surfaces of various layers following their deposition (see Hu et. al. in xe2x80x9cChemical-mechanical polishing as an enabling technology for giant magnetoresistance devicesxe2x80x9d, Thin Solid Films, 308-309 (1997), pp 555-561).
However, CMP has not been entirely satisfactory because the degree of smoothness required to achieve maximum performance of GMR devices is often beyond the capabilities of standard CMP processes since surface roughness on the order of a few Angstroms or less may be detrimental. Also, because CMP uses an abrasive slurry, it often leaves microscopic scratches in the polished surface.
It is therefore an object of this invention to facilitate the successful and precise smoothing of conductive films on insulating films or substrates.
It is a further object of this invention to provide a m smoothing of conductive films that is free of scratches.
It is a still further object of this invention to provide a m smoothing of conductive films that avoids damage to the magnetic and insulating films.
It is an even further object of this invention to provide a smoothing of conductive films that produces an ultra-smooth surface on a film of desired final thickness.
The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described hereinbelow.
The present invention facilitates the successful and precise smoothing of conductive films on insulating films or substrates. The system of this invention provides smoothing for metal films that is superior to prior chemical-mechanical polishing techniques and therefore results in a smoother surface that is substantially free of scratches. By supplying a source of low energy electrons, harmful charging of the films and damage to the magnetic and insulating films are avoided. Further characterizing the smoothing/thinning process in the present invention for a predetermined GCIB condition and using that characterization to determine an initial over-thickness of the starting film enables this process to produce a result that yields an ultra-smooth surface of desired final thickness.
For a better understanding of the present invention, together with other and further objects thereof, reference is made to the accompanying drawings and detailed description.